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As you try to find a few minutes each day to work out, you may wonder: What's the best time to exercise? There are some surprising pros and cons to exercising at different times.

Trainers often tout exercising first thing in the morning as ideal. Early birds do get their workout out of the way before the rest of their day begins, but research shows you're not at your physical peak until later in the day.

In each of us, an internal rhythm governs a host of physical systems. We call this roughly 24-hour cycle the circadian rhythm. For the most part, we're not conscious of this cycle. But you may have run afoul of your circadian rhythm when you feel jet lag. In that case, your internal clock isn't in sync with the world around you.

Less obvious are other body functions that shift through the day. "Pulmonary function, oxygen exchange, blood flow to the muscles, these are different at different times of the day," says Phyllis Zee, M.D., a professor of circadian biology at Northwestern University. "That makes your capacity for exercise different, too."

Afternoon is better

According to Dr. Zee, the body functions best in the late afternoon or early evening, roughly from 2 to 6 p.m. You reap some benefits by working out at this time. Among them: muscle strength is greater, muscles are warmer because of a slightly higher body temperature, lung function is improved and you even have a greater tolerance for discomfort.

Dr. Zee helped write a study that found people who exercised slept better than those who did not, and people who exercised later in the day had even more restful sleep. Still, she suggests you exercise at least two or three hours before you plan to go to sleep. Working out too close to bedtime can cause insomnia.

Although working out later may help you sleep and perform better, that doesn't mean it's the best time for everyone. Dr. Zee suggests you let your body, your level of alertness and your mood be your guide.

"People know whether they are a morning person or an evening person, whether they are a lark or an owl," she says. "If you are a lark, go ahead and exercise in the morning. If you are an owl, the better time for you to exercise is the late afternoon or early evening."

Researchers at the University of Surrey in England have found a genetic link between peoples' fondness for early morning or late evening. "If you have a night owl and you tell that person to work out in the morning, that's just not going to happen," says Carl Foster, Ph.D., a professor in the department of exercise and sports sciences at the University of Wisconsin-La Crosse.

A morning commitment

Early morning exercisers do have one decided advantage, says Carol Ann Kennedy, professor of kinesiology at Indiana University. "Exercise in the early morning tends to have higher adherence," she says. "That's the one time of the day when other obligations don't interfere."

Age also has a bearing on the best time to exercise. Young adults can exercise whenever they choose, with the time of day having little impact on their health. Older adults, however, may be at higher risk for stroke or heart attack when they exercise early in the day. An afternoon or an evening workout may be better for them.

Mealtimes also play a role. It's usually better not to exercise just after eating a large meal.

Even with these physiological factors, the most important question in choosing a time to exercise is this one: When will you be able to do it consistently?

"If you exercise, you're better off whenever you exercise," Dr. Foster says. "Even though exercising at a certain time may be 1 percent better, you need to find a time that fits for you as an individual and stick to it."